1. Field of Invention
The present invention relates to a printing device which is capable of improving print quality of a border portion of each print area, when the printing device prints an image by dividing it into a plurality of print areas.
2. Description of Related Art
Conventional printing devices print an image on a medium such as paper using patterns of dots by moving a print head in a row direction with respect to the paper. The print head is provided with a plurality of printing elements aligned in a column direction.
In such printing devices, a width printable by a single scan of the print head in the row direction is limited by the length in the column direction over which the printing elements are aligned. Thus, in order to print large graphic images or characters, scanning of the print head and paper feeding should be alternately repeated.
In this case, due to variations in pitch or insufficient mechanical rigidity of gears used in paper feed mechanism, several problems arise. Excessive paper feed will produce a clearance between print areas, which appears to be a white streak. Insufficient tape feed will cause the print areas to overlap with each other. The density of overlapping images is enhanced, and a black streak is produced.
Japanese Patent Publication No. 58-43028 and Japanese Patent Application Publication No. 60-168667 provide solutions for these problems.
Japanese Patent Publication No. 58-43028 is directed to a printing technique, in which printing is performed by jetting ink droplets from nozzles to paper. In an area where printing is repeated over a previously printed image, ink droplets are jetted from every other nozzle to prevent a border portion between the print areas from standing out.
Japanese Patent Application Publication No. 60-168667 is related to a printing technique, in which ink applied to a ink ribbon is melted by heating elements and transferred to paper. The print density of an image overlapping with a previously printed image is lowered to prevent a border portion between the print areas from standing out.
Both printing techniques disclosed in the above publications are effective when the paper feed mechanism operates perfectly without errors. In fact, however, it is impossible to operate the paper feed mechanism with perfection, as assembling or mechanical errors thereof cannot be eliminated. Thus, inconsistencies in density and changes in color are still noticeable in the border portion.
In view of the foregoing, one aspect of the invention is to provide a printing device which is capable of improving print quality of a border portion of each print area.
According to the invention, print quality of a border portion between print areas can be improved by shifting, before printing is repeated over a predetermined area of a previous print area, dots, which are smaller than dots printed in the previous print area, by a predetermined amount in a row direction from dots printed in the predetermined area,
Furthermore, before printing is repeated over a predetermined area of a previous print area, dots which are smaller than dots printed in the previous print area can be shifted by a predetermined amount in a column direction.
Furthermore, before printing is repeated over a predetermined area of a previous print area, dots which are smaller than dots printed in the previous print area can be shifted by a predetermined amount in row and column directions. In addition, color print quality of a border portion between print areas can be improved by changing, before printing is repeated over a predetermined area of a previous print area, the tone of dots to be printed according to the color used for printing.
In particular, it is preferable that the tone of dots to be printed is changed by selecting energy data which is associated with tone data for the color to be used for printing.
In addition, print quality of a border portion between print areas can be improved by printing at least two kinds of dots, which are different in area, over at least two kinds of dots, which are different in area and printed in a predetermined area of a previous print area.